In next generation wavelength division multiplexed (WDM) networks, an optical cross-connect (OXC) provides the capability of routing the optical path of multiple input/output fiber ports on different wavelengths or wavebands. In order for network management to control this reconfigurable function, the switching status should be set prior to each data transmission. Since the optical connections for WDM signal transmission are strongly dependent on the switching status and switching quality of OXC, switching failure or malfunction of OXC can lead the data stream to an incorrect destination, cause a collision with another signal, degrade signal performance, and cause a loss of live traffic.
The current state of the art on switching status monitoring includes the use of either in-band pilot tone technology (used by Nortel Networks and documented in Hamazumi, et al JLT15, p. 2197, 1997)) or local out-band ID signal generation and detecting (documented in Chang, et al., PTL6, p. 899, 1998 and Zhong et al., Digest OFC'2000).
Wavelength selective switches (WSS)(an OXC with granularity of single wavelength), for example an N×N WSS, have been widely proposed and studied in the last few years as a cost-effective solution to provide a transparent by-pass for WDM express traffic at degree n nodes in optical networks. WSS's provide an optical cross-connect function with single channel granularity, where any WDM channel from any of the N inputs can be routed to any of the N outputs.
Until recently, the implementation of WSS in commercial systems was limited by the maturity of optical components and ultra-long haul optical transport technology. With these technologies now becoming available, there is a need to consider additional challenges associated with network monitoring and node management in the optical layer.
Current optical performance monitoring (OPM) solutions just basically monitor optical properties of existing channels along the transmission line. Monitoring of WSS, however, is more essential and demanding. Beyond the general physical layer monitoring, such as the insertion loss profile, cross-talk, etc., which affects the quality of signals passing through the WSS, there is no general capability to verify the connectivity of WSS, even before new traffic signals are provisioned in order to establish that a particular optical circuit is available and to avoid potential wavelength collisions downstream.
Current optical performance monitoring technology cannot satisfy these requirements, and thus an in-service, traffic signal independent monitoring schemes are desirable.